In recent years, regulations on CO2 emissions have been tightened in awareness of environmental issues. In the automotive field, the improvement of fuel efficiency by automotive weight reduction is a big issue. Therefore, gauge reduction by applying high-strength steel sheets to automotive parts has been pursued. Steel sheets with a TS of 590 MPa or more are applied to parts for which steel sheets with a TS of 270 MPa to 440 MPa have been conventionally used.
The steel sheets with a TS of 590 MPa or more need to have properties being excellent in formability typified by ductility and stretch flange formability (hole expansibility) from the viewpoint of formability and also being high in impact energy absorbing capability. An increase in yield ratio is effective in order to enhance impact energy absorbing capability and it enables impact energy to be efficiently absorbed even with a small amount of deformation.
From the viewpoint of mechanisms for strengthening a steel sheet to achieve a tensile strength of 590 MPa or more, there is a method making use of hardening of a ferrite phase, which is a parent phase, and there is another one making use of a hard phase such as a martensite phase. As for hardening of a ferrite phase, precipitation-hardened high-strength steel sheets containing a carbide-forming element such as Nb can be produced at low cost because the amount of an alloying element necessary to achieve a predetermined strength is small.
For example, Patent Literature 1 discloses a method for producing a galvanized steel sheet, precipitation-hardened by the addition of Nb, having a tensile strength of 590 MPa or more and excellent resistance to secondary working embrittlement after press forming. Patent Literature 2 discloses a high-strength cold-rolled steel sheet, precipitation-hardened by the addition of Nb and Ti, having a tensile strength TS of 490 MPa to less than 720 MPa, a yield ratio of more than 0.70 to less than 0.92, excellent stretch flange formability, and excellent impact energy absorbing capability and also discloses a method for producing the same. Patent Literature 3 discloses a high-strength cold-rolled steel sheet, precipitation-hardened by the addition of one or both of Nb and Ti, having high yield ratio. This steel sheet has a microstructure containing recrystallized ferrite, unrecrystallized ferrite, and pearlite; a maximum tensile strength of 590 MPa or more; and a yield ratio of 0.70 or more.
On the other hand, as for a method making use a hard phase such as a martensite phase, for example, Patent Literature 4 discloses a dual-phase high-strength cold-rolled steel sheet having excellent dynamic deformability due to a multi-phase microstructure containing a primary phase which is ferrite, a secondary phase containing 3% to 50% martensite on a volume fraction basis, and other low-temperature transformation phases and also discloses a method for producing the same. Patent Literature 5 discloses a high-strength steel sheet having excellent stretch flange formability and crashworthiness. The high-strength steel sheet is composed of a ferrite phase which is a primary phase and a martensite phase which is a secondary phase, the martensite phase being a maximum grain size of 2 μm or less and an area fraction of 5% or more.